- Email: [email protected]
CH3O(X 2E) production from 266 nm photolysis of methyl nitrite and reaction with NO
Chemical Physics 49 (1980) 17-22 0 North-Holland Publishing Company
CH,0(%2E)PRODUCTION FROM266nmPHOTOLYSIS OFMETHYLNlTRITEANDREACTiONWlTHNO N. SANDERS *, J.E. BUTLER, L.R. PASTERNACK and J.R. MCDONALD Loser Clremistry Group, Cizemistry Division, Naval Research Laboratory, Ikhtigton,
DC20375.
Received 2 January
USA
1980
The CHs0(z2E) radial produced by the 266 nm photolysis of CHaONO is chkcterized by laser-induced fluorescence. Using a flowing gas cell the reaction rate of CH30(%zE) with NO is measured to be (2.08 c 0.12) X IO-” cm3 s-l molecule-’ based upon disappearance of CH30 and appearance of HNO detected by laser induced fluorescence. Upper limits for CHsO reactions with CH4, CO, N20, NH3, CHzOH, (CH&CH and’CH2=CHCHzCH3 are reported. These reactions ye all too slow to measure under OUTexperimental conditions.
1.Introduction The oxidation of hydrocarbons in flames and the upper and lower atmosphere is effected by reactions involving the methoxy radical (CH30). The understanding of tropospheric photochemistry has particularly hinged upon the chemical kinetic behavior of organic free radicals, including alkoxy radicals [ I] _ Studies of methoxy reactions have been hampered by the lack of direct detection techniques. Competitive reaction methods have been almost the so!e origin of the currentIy available rate data. The most important rates, those with 02 and hydrocarbons, have been measured by several different investigators [2-81 using various methoxy sources (both photoIytic and pyrolytic) over a temperature range of 25’C -250°C. These data are summarized in table 1 along with constants determined for CO [9] and CH2=CHZ [lo]. Rates of reaction of formyl derivatives (HCO-) have also been studied [7,11,12]. The recent observation of laser-excited fluorescence from methoxy radicals [ 131 suggests a technique for directly monitoring their concentration during chemical reactions. The experiments reported here use the photodissociation of methyl nitrite (CH,ONO) vapor by pulsed laser radiation at 266 nm * NRC/NRL Postdoctoral
Research Associate.
.
to generate CH30 which is then probed by a second W laser pulse at a variabIe time delay after the photolysis. A number of similar single and multi-photon photolysis experiments performed in this laboratory [14] have studied the spectroscopy and reactivity of unstable species important in combustion systems [e.g. -CN, C2, $0, CH, O(‘D), OH]. In cases where an added reactant gas removes CH,O faster than its loss due to various other factors (including simple diffusion out of the probe laser volume), pseudofirst order rate constants for CH30 disappearance can be accurately measured. We report such measurements for the reaction of NO with methoxy radicals and limiting rate constants for reactions with other gases which are too slow to measure directly with our experimental apparatus. In addition, HNO has been observed as a secondary product via its laser-excited fludrescence spectrum.
2. Experimental Fig. 1 shows the experimental apparatus used. Similar two-laser arrangements designed in this laboratory for both spectroscopy and kinetics of photofragments have been previously described [14]. Briefly, the quadrupled output from a Nd : YAG laser (266 nm, 3 mJ/pulse, 10 Hz, 15 ns fwhm, Quantel Int’l
I’?. Sanders et at. jCII,O(~
18
‘E) production fioin photolysisof
Cff~ONO
Table 1 Summary of CHsO kinetic data
Gas
Maximum
k
(em3 molecule-’ s-l)
This work log k (hl-* s-l)
Other measurements
preWl1a used florr)
(2.08 * 0.12) x IO-” <2 x 10s
10.10 c 0.02 C6.0
<6.7 <6.7
10.2 r 0.6 a) gc) 5.5 d, 6.10 0.3 9) 1.7 f 0.6 h)
<7-o c7.7 <7.7 C&O ~8.3
1.5 e 0.3 i) ]I
NO SO
02
10 10 S 1 1
CH4
CO N20 NH3
CH,OH (CH313CH
CEi2=CHCH2CH3 CHZO
a) Ref. [2]. 3 Ref. 121.
<1
x 10-14
0.5 0.25
x
lo-‘3
<2
x
10-13
<4 x lo-” -
-
7.4 b) 6.1e)
4.9 k,
p) Ref. [22] c, Ref. [21. d, Ref. [4]_ e, Ref. [3] (at =lOO”C). 0 Ref. [23]. J) Ref. [IO]: at 12?C CHz=CHz reacts 4.6 + 0.5 times faster than CO. k, Ref. [7].
g) Ref. [6].
h, Ref. [9].
YG 482 CM) is separated from the 1064 nm fundamental by a dielectric beam-splitter and from the 532 run first harmonic by a Pellin-Broca prism. The 266 nm beam is telescoped to ~2 mm diameter and direct-
34400 cm-l) is focused antiparallel to the photolysis beam at iris A. The near W laser-induced fluorescence from CH,O radicals at the center of the cell is collected by a 1 : 1 telescope, passes filters (Schott W-32
ed along the axis of the gas cell. A frequency doubled, pulsed, flashlamp pumped dye laser (Chromatix CMX-4) which provides the probe beam (32 ZOO-
and UG-11) and is focused onto the cathode of an RCA 31000 A photomultiplier. For HNO laser-induced fluorescence, the fundamen-
PUMPING
; 1 CAPACITrVJCE
INTERFACE
Fii. 1. Experimental arrangement for measuring CM30 kinetics: L: 1 m focal Length quartz lens; B, A: 2 mm diameter ties; T*: te1esoope-n : m beam reduction; F: filters; BS: dielectric Seam splitter; SHG: second harmonic generation uystal; D: pulse delay
generator.
N. Sanders et al./cH,Ofy
2E) production from photo[ysis of C&ON0
19
33000
33500 LASER
FREQUENCY
(cm-‘)
Fig. 2. Laser-induced fluorescence excitation spectrum of CHsO showing I& = 2 + 0, and 3 6 0 vlbronic bands. Relativeenergy of the dye laser is given by the upper trace - no corrections have bwn made for detection system response.
tal output of oxazine 720 and rhodamine 640 dyes (13 900-l 5 700 cm-’ j is used and the near-IR fluorescence is monitored by an RCA 3 1034 photomultiplier. Methyl nitrite is synthesized ’ by the slow addition of a mixture of 20 m! nitrosylsulfuric acid (HOSO, ONO, American Hoechst) and 20 ml concentrated sulfuric acid to 250 ml of a 20% volume : volume solution of methanol in water under a helium atmosphere. The product vapours are carried over by a slow helium flow and condensed at -196°C. Following completion of the reaction the products are separated by trap to trap distillation. CH,ONO is collected at -13O’C as a pale yellow, glassy solid. IR spectra reveal a trace contamination (
route.
concentration (see fig. 1). Pressures are measured with a Baratron model 170 M capacitance manometer. SF6 is used as a buffer gas in these experiments for several reasons. Its presence prevents diffusion out of the detection region during the experimental measurement time. In-additio?, SF, cools the nascent CH30 radicals and stabilizes their initial concentration level 02 a time scale short (
3. Results 3.1. Production of CH3U The laser induced fluorescence excitation spectrum of the CH30 product of CH30N0 photolysis is shown in fig_ 2. Little or no change is observed in the excitation spectrum as the delay between the probe and photolysis pulses is varied from OS-400 ,us. A similar excitation spectrum to that shown in fig. 2 was reported by
20
N. Swzders er oL/CH30(z2E)
production from photolysis of CHSONO
Inoue et al. [ 131 folIowing the reaction of F atoms with CH30H. ‘Re CH30-NO bond strength in CH30N0 is ~37 kcal/mole [I 71 so that photolysis at 266 nm leaves the fragments with =70 kcal/mole of excess energy to be distributed between translation and internal degrees of freedom (vibration-rotation)of the products. At very low pressures of CH, ON0 (3 mTorr) the band a: 33 500 cm-l (previously assigned [ 13,I 61 as A2A1, ~5 = 3 + X2E, u; = 0) has a risetime of 1.5 ps at 0.5 Torr pressure of SF6. The rise time becomes unmeasurable (<0.5 ps) at SF6 pressures of IO Torr or higher. The intensity of the CH,O laser-induced lluorescence signal increases greatly (1 O-20 fold) over this range (factor of 20) of buffer gas pressure. Similar behavior Is noted with Ar and N2 as buffer gases. Under the conditions of the kinetics experiments (CH30N0 pressure IO-80 mTorr, buffer gas pressure IO-SO Torr) the CH30 disappearance rate is 350 ? 50 ps and shows no obvious dependence on the type or pressure of buffer gas or the CH,ONO pressure. The laser-induced fluorescence intensity depends lineariy upon photolysis laser power up to the onset of saturation (-4 mJ/puIse at a peak power of ~5 MW/cm’ at 40 mTorr CH3 ONO). We find no strong evidence for the u3 = 1 hot bands tentatively assigned by Inoue et al. [ 131 from our (weak) spectra taken under conditions where the ground state (us = 0) spectrum has an observable risetime. The observed CH30 (‘E, u”= 0) concentration increase induced by buffer gas collisions is too large to be attributed to vibrational and rotational relaxation of ground state CH,O populations. There are several other possibIe explanations consistent with our observations: (a) collision induced intersyste,m crossing from an unidentified quartet metastable CH,O state produced in the CH30N0 photolysis followed by a rapid relaxation to IJ” = 0, ‘E populations; (b)coIIision induced dissociation of CH30NO* populations which have an interna energy higher than the dissociation limit of x37 kcal/mole; (c) a collision induced isomerization of other primary radicals possibly produced in the photolysis of CH,ONO. This might include such species as CH20H which is known to be nearly isoenergetic with CH30 1161. Our experiments
do not distinguish among these
possible mechanisms. For the purposes of this report it is sufficient to observe that the CHsO(*E, ~“‘0) population is created and stabilized in GO0 ns after the photolysis pulse under our experimental conditions. 3.2. Reactim of Cff30 with NO
Of the species investigated the only reactant which has a rate constant large enough at 22°C to be measurable against the background decay is NO. The possible channels and approximate enthalpies at 25°C are [17] AH (kcaI/moIe) CH30 + NO 2 CH,ONOF
-37
(1)
+ CH20 -I-HNO,
-24
(2)
+CH,
+19.
(3)
+NO,,
The third channel is very improbable at room temperature. Batt et al. [z] have reviewed previous determinations of the branching ratio k2/kl in the high pressure limit and concluded that the best value is 0.17 with neither rate constant having any observable temperature dependence from 20-200°C.
Values of the combined decay constant (kl C k2) in the high pressure limit measured by various indirect methods are given in table 1 along with our value. Sample pseudo first-order plots are shown in fig. 3 and the second order plot in fig. 4. The lowest NO pressure measured gives a CH30 decay rate about four times faster than that observed without NO. The laser induced fluorescence excitation spectrum of HNO was observed in conjunction with the study of the NO f CH;O reaction. LIF spectra of HNO have also been reported by Yamada et al. [ 181 using a flowing discharge. Dalby [I91 obtained the absorption spectrum by UV photoIysis of 0.5 Torr isoamyl &rite. Accordingly, in our experiments the strongest features of this spectrum could just be detected via the near IR induced fluorescence using 40 mTorr CH,ONO and no added NO. With added NO the HNO LIF signal is = 10 times stronger and three vibronic bands have been extensively mapped out. A portion of the ~lA”(O10) -+%lA’(OOO) band (detecting X> 740 nm) appears in fig. 5. The (001) + (000) (X,, = 710 run, X,, > 740 m-n) and the (0 11) + (000) (&xc =
N:&nde~~ et ai./CH30f_? 2EJ production from photoIysi3 of CH3ONO
20
21
60
100
P,,(mlorr)
Fig. 4. E’lot of pseudo-fist order rate constants tant pressure in the CH30 + NO reaction k’ = 7- and
100
50
k’ = 3 subbands of the AK = +l (010) +
(300) absorption
t(pecl
Fii. 3. SampIe plots of CHBO pseudo-fust order decays observed in the CH90 c NO reaction. Vertid scale is Iqarithmic.
6-40nm,X&s2 660 nm) bands are much weaker. Risetimes of the LIF HNO signals at 14 670 f 2.0 cm -’ and 14730 i 20 cm-t (corresponding to the
against reac-
band [ 191) as functions
of added NC
are in agreement with the CH30 disappearance rates. Direct production of HNO from CH,ONO CH,ONO*
+ HNO + CH,O.
(4)
as orignally proposed by Puikis and Thompson [20] and later supported by Brown and Pimentel [21] is at least t&n times less important
than the dissociation
i
145Co
LPSER
FREMRNCl
fluorescence excitation spectrum of ment of major features follows Dalby [IQ].
F& 5. Laser-induced
(CM-‘)
HNO exciting in the A ‘A”(Ol0)
r5ooo
+X ‘A’(OO0) absorption band. AS&-
22
N. Sanders et cl./CH,O(~
2Z) production
channel leading ‘to CHSO. No attempt was made to follow the detailed structure of the HNO spectrum in time but it appears that rotationaT and vibrational distributions of HNO produced by CH,O + NO could be measured under the proper conditions. 3.3. Reactiotz of C!!sO with otizerguses
It is not possible for us to measure the rate of reaction of CH30 with the other gases studied. The maximum pressure of reactant gas used is limited by loss of the methoxy LIF signat. Ln most cases this loss can likely be attributed to quenching of the CH,O state (x “A,) prepared by the probe laser. Since the ma?rimum reactive gas pressure given in table 1 represcnts the point at which the initiaf LIF fluorescence signal is reduced to 2-4% of its value with no added gas, the relative quenching rates may then be inferred from the pressure ratios. Ohbayashi et al. [I.51 measured the quenching of CH30* emission following W photoIysis of CH30N0. Relative rates extracted from their data for 02, CH4, CO, N2 and Ar are in agreement with our (much cruder) observations. The high apparent quenching rates for I-butene and isobutane (~40 and ~30 times that of methane) cornpared to those measured for ethylene and propane (both ~3 times methane) may be justified on grounds of the stability of the radical abstraction product. It is not possible to distinguish between electronic quenching and reaction of the CH,O x*A, state in our experiments. The upper limits for reaction rates of CHSO @‘E, u’; = 0) are estimated by comparison of the maximum reactant pressure (observed decay rate less than twice the background decay) with the NO pressure needed to double the background decay rate (2.5 mTorr). These data are summarized in table 1 and are consistent with measurements by other workers where they are available.
References [II Chemical Kinetic Data Needs for Modeling the Lower Troposphere, NBS Special Pubbution g557 (US Department of Commerce, Washington, 1979). [2] L. Batt, R.T. Miiae and R.D. McCuUoch, Intern. J. Chem. Kinet. 9 (1977) 567;
fmm photolysis
of CHsONO
L. Batt and R.D. hfcCulloch, Intern. J. Chem. Kinet. 8 (1976) 491. [3] W.G. Aicock and B. Mile, Combust. Flame 29 (1977) 133. [4] C.D. Mendenhail, D.M. Golden and S.W. Benson, Intern. I. Chem. Kinet. 7 (1975) 725. [S] H.A. Wiebe and J. Heicklen, J. Am. Chem. Sot 95 (1973) 1; HA Wiebe, A. ViUa, T.M. Heliman and I. Heickien, J. Am. Chem. Sot. 95 (1973) 7. [6] R. Shaw and J.C.J. Thynne, Trans. Faraday Sot. 62 (1966) 104. [7] D.E. Hoare and C.A. Wellington, Proceedings of the 8th Intemationht Symposium on Combustion (Wiim & Wiiins. Baltimore, 1962). [S] T. Berces and A.F. Trotman-Dickenson, J. Chem. Sot. (1961) 384; R. Shaw and A.F. Trotman-Dickenson, J. Chem. Sot. (1960) 3210. [9] Ed. Lissi. C. Massiff and A.E. Viiia, J. Chem. Sot. Faraday Trans. I 69 (1973) 346. [IO] E.A. Lissi, G. Massiff and A.E. Vi. Intern. J. Chem. Kinet. 7 (1975) 625. [ 1 l] J.C.J. Thynne and P. Gray, Trans. Faraday Sot. 59 (1963) 1149. [12] N. Kelly and J. Heicklen, J. Photochem. 8 (1978) 83; B.A. DeCraff and J.G. Calvert, J. Am. Chem. Sot. 89 (1967) 2247. [13] G. Inoue. H. Akimoto and M. Okuda, Chem. Phys. Letters 63 (1979) 213; Abstracts of the ‘4th International Symposium on Free Radicals, Sanda. Hyogo-ken. Japan, 3-7 September i979, pp. 103-105. [14] A.P. Baronavski and J.R. hIcDonald, Chem. Phys. Letters 45 (1977) 172; V.M. Donnelly and L. Pastemack;Chem. Phys. 39 (1979) 427: A.P. Baronavski and J.R W.M. Pitt% V.M. DoMeUY, McDonald, to be published; JE. Butler, L.P. Goss, hi.C. Lin and J.W. Hudgens, Chem. Phys. Letters 63 (1979) 104; G.K. Smith, JB. Butler and MC. Lin, Chem. Phys. Letters 6.5 (1979) 115. [15] K. Ohbayashb H. Akimoto and I. Tanaka, J. Phys. Chem. 81 (1977) 798. [16] H.E. Hunziker. Abstracts of 14th International Symposium on Free Radicals, Sanda, Hyogo-ken, Japan, 3-7 September 1979, pp_ 84-87. [17] J-G. Calvert and J.N. Pitts, Photochemistry (Wiley, New York, 1966). [ 181 F. Yamada. T. Ishiwata, M. Kawasaki, K. Obi and I. Tanaka, J. Photochem. 9 (1978) 309. [19] F.W. Daiby, Can. J. Phys. 36 (1958) 1336. [20] C.H. Purkis and H.W. Thompson, Trans. Faraday Sot. 32 (1936) 1466. [21] H.W. Brown and C.C. Pimentel. J. Chem. Phys. 29 (1958) 883. [22] EA. Arden, L. Philhps and R. Shaw, J. Chem. Sot (1964) 5126. [231 H.S. Johnston, JN. Pitts. S. Lewis, L Zafonte and T. Mottersnead, Project Clean Air Task Force No. 7, University of California, 1970, pp. 3-6.
CH,0(%2E)PRODUCTION FROM266nmPHOTOLYSIS OFMETHYLNlTRITEANDREACTiONWlTHNO N. SANDERS *, J.E. BUTLER, L.R. PASTERNACK and J.R. MCDONALD Loser Clremistry Group, Cizemistry Division, Naval Research Laboratory, Ikhtigton,
DC20375.
Received 2 January
USA
1980
The CHs0(z2E) radial produced by the 266 nm photolysis of CHaONO is chkcterized by laser-induced fluorescence. Using a flowing gas cell the reaction rate of CH30(%zE) with NO is measured to be (2.08 c 0.12) X IO-” cm3 s-l molecule-’ based upon disappearance of CH30 and appearance of HNO detected by laser induced fluorescence. Upper limits for CHsO reactions with CH4, CO, N20, NH3, CHzOH, (CH&CH and’CH2=CHCHzCH3 are reported. These reactions ye all too slow to measure under OUTexperimental conditions.
1.Introduction The oxidation of hydrocarbons in flames and the upper and lower atmosphere is effected by reactions involving the methoxy radical (CH30). The understanding of tropospheric photochemistry has particularly hinged upon the chemical kinetic behavior of organic free radicals, including alkoxy radicals [ I] _ Studies of methoxy reactions have been hampered by the lack of direct detection techniques. Competitive reaction methods have been almost the so!e origin of the currentIy available rate data. The most important rates, those with 02 and hydrocarbons, have been measured by several different investigators [2-81 using various methoxy sources (both photoIytic and pyrolytic) over a temperature range of 25’C -250°C. These data are summarized in table 1 along with constants determined for CO [9] and CH2=CHZ [lo]. Rates of reaction of formyl derivatives (HCO-) have also been studied [7,11,12]. The recent observation of laser-excited fluorescence from methoxy radicals [ 131 suggests a technique for directly monitoring their concentration during chemical reactions. The experiments reported here use the photodissociation of methyl nitrite (CH,ONO) vapor by pulsed laser radiation at 266 nm * NRC/NRL Postdoctoral
Research Associate.
.
to generate CH30 which is then probed by a second W laser pulse at a variabIe time delay after the photolysis. A number of similar single and multi-photon photolysis experiments performed in this laboratory [14] have studied the spectroscopy and reactivity of unstable species important in combustion systems [e.g. -CN, C2, $0, CH, O(‘D), OH]. In cases where an added reactant gas removes CH,O faster than its loss due to various other factors (including simple diffusion out of the probe laser volume), pseudofirst order rate constants for CH30 disappearance can be accurately measured. We report such measurements for the reaction of NO with methoxy radicals and limiting rate constants for reactions with other gases which are too slow to measure directly with our experimental apparatus. In addition, HNO has been observed as a secondary product via its laser-excited fludrescence spectrum.
2. Experimental Fig. 1 shows the experimental apparatus used. Similar two-laser arrangements designed in this laboratory for both spectroscopy and kinetics of photofragments have been previously described [14]. Briefly, the quadrupled output from a Nd : YAG laser (266 nm, 3 mJ/pulse, 10 Hz, 15 ns fwhm, Quantel Int’l
I’?. Sanders et at. jCII,O(~
18
‘E) production fioin photolysisof
Cff~ONO
Table 1 Summary of CHsO kinetic data
Gas
Maximum
k
(em3 molecule-’ s-l)
This work log k (hl-* s-l)
Other measurements
preWl1a used florr)
(2.08 * 0.12) x IO-” <2 x 10s
10.10 c 0.02 C6.0
<6.7 <6.7
10.2 r 0.6 a) gc) 5.5 d, 6.10 0.3 9) 1.7 f 0.6 h)
<7-o c7.7 <7.7 C&O ~8.3
1.5 e 0.3 i) ]I
NO SO
02
10 10 S 1 1
CH4
CO N20 NH3
CH,OH (CH313CH
CEi2=CHCH2CH3 CHZO
a) Ref. [2]. 3 Ref. 121.
<1
x 10-14
0.5 0.25
x
lo-‘3
<2
x
10-13
<4 x lo-” -
-
7.4 b) 6.1e)
4.9 k,
p) Ref. [22] c, Ref. [21. d, Ref. [4]_ e, Ref. [3] (at =lOO”C). 0 Ref. [23]. J) Ref. [IO]: at 12?C CHz=CHz reacts 4.6 + 0.5 times faster than CO. k, Ref. [7].
g) Ref. [6].
h, Ref. [9].
YG 482 CM) is separated from the 1064 nm fundamental by a dielectric beam-splitter and from the 532 run first harmonic by a Pellin-Broca prism. The 266 nm beam is telescoped to ~2 mm diameter and direct-
34400 cm-l) is focused antiparallel to the photolysis beam at iris A. The near W laser-induced fluorescence from CH,O radicals at the center of the cell is collected by a 1 : 1 telescope, passes filters (Schott W-32
ed along the axis of the gas cell. A frequency doubled, pulsed, flashlamp pumped dye laser (Chromatix CMX-4) which provides the probe beam (32 ZOO-
and UG-11) and is focused onto the cathode of an RCA 31000 A photomultiplier. For HNO laser-induced fluorescence, the fundamen-
PUMPING
; 1 CAPACITrVJCE
INTERFACE
Fii. 1. Experimental arrangement for measuring CM30 kinetics: L: 1 m focal Length quartz lens; B, A: 2 mm diameter ties; T*: te1esoope-n : m beam reduction; F: filters; BS: dielectric Seam splitter; SHG: second harmonic generation uystal; D: pulse delay
generator.
N. Sanders et al./cH,Ofy
2E) production from photo[ysis of C&ON0
19
33000
33500 LASER
FREQUENCY
(cm-‘)
Fig. 2. Laser-induced fluorescence excitation spectrum of CHsO showing I& = 2 + 0, and 3 6 0 vlbronic bands. Relativeenergy of the dye laser is given by the upper trace - no corrections have bwn made for detection system response.
tal output of oxazine 720 and rhodamine 640 dyes (13 900-l 5 700 cm-’ j is used and the near-IR fluorescence is monitored by an RCA 3 1034 photomultiplier. Methyl nitrite is synthesized ’ by the slow addition of a mixture of 20 m! nitrosylsulfuric acid (HOSO, ONO, American Hoechst) and 20 ml concentrated sulfuric acid to 250 ml of a 20% volume : volume solution of methanol in water under a helium atmosphere. The product vapours are carried over by a slow helium flow and condensed at -196°C. Following completion of the reaction the products are separated by trap to trap distillation. CH,ONO is collected at -13O’C as a pale yellow, glassy solid. IR spectra reveal a trace contamination (
route.
concentration (see fig. 1). Pressures are measured with a Baratron model 170 M capacitance manometer. SF6 is used as a buffer gas in these experiments for several reasons. Its presence prevents diffusion out of the detection region during the experimental measurement time. In-additio?, SF, cools the nascent CH30 radicals and stabilizes their initial concentration level 02 a time scale short (
3. Results 3.1. Production of CH3U The laser induced fluorescence excitation spectrum of the CH30 product of CH30N0 photolysis is shown in fig_ 2. Little or no change is observed in the excitation spectrum as the delay between the probe and photolysis pulses is varied from OS-400 ,us. A similar excitation spectrum to that shown in fig. 2 was reported by
20
N. Swzders er oL/CH30(z2E)
production from photolysis of CHSONO
Inoue et al. [ 131 folIowing the reaction of F atoms with CH30H. ‘Re CH30-NO bond strength in CH30N0 is ~37 kcal/mole [I 71 so that photolysis at 266 nm leaves the fragments with =70 kcal/mole of excess energy to be distributed between translation and internal degrees of freedom (vibration-rotation)of the products. At very low pressures of CH, ON0 (3 mTorr) the band a: 33 500 cm-l (previously assigned [ 13,I 61 as A2A1, ~5 = 3 + X2E, u; = 0) has a risetime of 1.5 ps at 0.5 Torr pressure of SF6. The rise time becomes unmeasurable (<0.5 ps) at SF6 pressures of IO Torr or higher. The intensity of the CH,O laser-induced lluorescence signal increases greatly (1 O-20 fold) over this range (factor of 20) of buffer gas pressure. Similar behavior Is noted with Ar and N2 as buffer gases. Under the conditions of the kinetics experiments (CH30N0 pressure IO-80 mTorr, buffer gas pressure IO-SO Torr) the CH30 disappearance rate is 350 ? 50 ps and shows no obvious dependence on the type or pressure of buffer gas or the CH,ONO pressure. The laser-induced fluorescence intensity depends lineariy upon photolysis laser power up to the onset of saturation (-4 mJ/puIse at a peak power of ~5 MW/cm’ at 40 mTorr CH3 ONO). We find no strong evidence for the u3 = 1 hot bands tentatively assigned by Inoue et al. [ 131 from our (weak) spectra taken under conditions where the ground state (us = 0) spectrum has an observable risetime. The observed CH30 (‘E, u”= 0) concentration increase induced by buffer gas collisions is too large to be attributed to vibrational and rotational relaxation of ground state CH,O populations. There are several other possibIe explanations consistent with our observations: (a) collision induced intersyste,m crossing from an unidentified quartet metastable CH,O state produced in the CH30N0 photolysis followed by a rapid relaxation to IJ” = 0, ‘E populations; (b)coIIision induced dissociation of CH30NO* populations which have an interna energy higher than the dissociation limit of x37 kcal/mole; (c) a collision induced isomerization of other primary radicals possibly produced in the photolysis of CH,ONO. This might include such species as CH20H which is known to be nearly isoenergetic with CH30 1161. Our experiments
do not distinguish among these
possible mechanisms. For the purposes of this report it is sufficient to observe that the CHsO(*E, ~“‘0) population is created and stabilized in GO0 ns after the photolysis pulse under our experimental conditions. 3.2. Reactim of Cff30 with NO
Of the species investigated the only reactant which has a rate constant large enough at 22°C to be measurable against the background decay is NO. The possible channels and approximate enthalpies at 25°C are [17] AH (kcaI/moIe) CH30 + NO 2 CH,ONOF
-37
(1)
+ CH20 -I-HNO,
-24
(2)
+CH,
+19.
(3)
+NO,,
The third channel is very improbable at room temperature. Batt et al. [z] have reviewed previous determinations of the branching ratio k2/kl in the high pressure limit and concluded that the best value is 0.17 with neither rate constant having any observable temperature dependence from 20-200°C.
Values of the combined decay constant (kl C k2) in the high pressure limit measured by various indirect methods are given in table 1 along with our value. Sample pseudo first-order plots are shown in fig. 3 and the second order plot in fig. 4. The lowest NO pressure measured gives a CH30 decay rate about four times faster than that observed without NO. The laser induced fluorescence excitation spectrum of HNO was observed in conjunction with the study of the NO f CH;O reaction. LIF spectra of HNO have also been reported by Yamada et al. [ 181 using a flowing discharge. Dalby [I91 obtained the absorption spectrum by UV photoIysis of 0.5 Torr isoamyl &rite. Accordingly, in our experiments the strongest features of this spectrum could just be detected via the near IR induced fluorescence using 40 mTorr CH,ONO and no added NO. With added NO the HNO LIF signal is = 10 times stronger and three vibronic bands have been extensively mapped out. A portion of the ~lA”(O10) -+%lA’(OOO) band (detecting X> 740 nm) appears in fig. 5. The (001) + (000) (X,, = 710 run, X,, > 740 m-n) and the (0 11) + (000) (&xc =
N:&nde~~ et ai./CH30f_? 2EJ production from photoIysi3 of CH3ONO
20
21
60
100
P,,(mlorr)
Fig. 4. E’lot of pseudo-fist order rate constants tant pressure in the CH30 + NO reaction k’ = 7- and
100
50
k’ = 3 subbands of the AK = +l (010) +
(300) absorption
t(pecl
Fii. 3. SampIe plots of CHBO pseudo-fust order decays observed in the CH90 c NO reaction. Vertid scale is Iqarithmic.
6-40nm,X&s2 660 nm) bands are much weaker. Risetimes of the LIF HNO signals at 14 670 f 2.0 cm -’ and 14730 i 20 cm-t (corresponding to the
against reac-
band [ 191) as functions
of added NC
are in agreement with the CH30 disappearance rates. Direct production of HNO from CH,ONO CH,ONO*
+ HNO + CH,O.
(4)
as orignally proposed by Puikis and Thompson [20] and later supported by Brown and Pimentel [21] is at least t&n times less important
than the dissociation
i
145Co
LPSER
FREMRNCl
fluorescence excitation spectrum of ment of major features follows Dalby [IQ].
F& 5. Laser-induced
(CM-‘)
HNO exciting in the A ‘A”(Ol0)
r5ooo
+X ‘A’(OO0) absorption band. AS&-
22
N. Sanders et cl./CH,O(~
2Z) production
channel leading ‘to CHSO. No attempt was made to follow the detailed structure of the HNO spectrum in time but it appears that rotationaT and vibrational distributions of HNO produced by CH,O + NO could be measured under the proper conditions. 3.3. Reactiotz of C!!sO with otizerguses
It is not possible for us to measure the rate of reaction of CH30 with the other gases studied. The maximum pressure of reactant gas used is limited by loss of the methoxy LIF signat. Ln most cases this loss can likely be attributed to quenching of the CH,O state (x “A,) prepared by the probe laser. Since the ma?rimum reactive gas pressure given in table 1 represcnts the point at which the initiaf LIF fluorescence signal is reduced to 2-4% of its value with no added gas, the relative quenching rates may then be inferred from the pressure ratios. Ohbayashi et al. [I.51 measured the quenching of CH30* emission following W photoIysis of CH30N0. Relative rates extracted from their data for 02, CH4, CO, N2 and Ar are in agreement with our (much cruder) observations. The high apparent quenching rates for I-butene and isobutane (~40 and ~30 times that of methane) cornpared to those measured for ethylene and propane (both ~3 times methane) may be justified on grounds of the stability of the radical abstraction product. It is not possible to distinguish between electronic quenching and reaction of the CH,O x*A, state in our experiments. The upper limits for reaction rates of CHSO @‘E, u’; = 0) are estimated by comparison of the maximum reactant pressure (observed decay rate less than twice the background decay) with the NO pressure needed to double the background decay rate (2.5 mTorr). These data are summarized in table 1 and are consistent with measurements by other workers where they are available.
References [II Chemical Kinetic Data Needs for Modeling the Lower Troposphere, NBS Special Pubbution g557 (US Department of Commerce, Washington, 1979). [2] L. Batt, R.T. Miiae and R.D. McCuUoch, Intern. J. Chem. Kinet. 9 (1977) 567;
fmm photolysis
of CHsONO
L. Batt and R.D. hfcCulloch, Intern. J. Chem. Kinet. 8 (1976) 491. [3] W.G. Aicock and B. Mile, Combust. Flame 29 (1977) 133. [4] C.D. Mendenhail, D.M. Golden and S.W. Benson, Intern. I. Chem. Kinet. 7 (1975) 725. [S] H.A. Wiebe and J. Heicklen, J. Am. Chem. Sot 95 (1973) 1; HA Wiebe, A. ViUa, T.M. Heliman and I. Heickien, J. Am. Chem. Sot. 95 (1973) 7. [6] R. Shaw and J.C.J. Thynne, Trans. Faraday Sot. 62 (1966) 104. [7] D.E. Hoare and C.A. Wellington, Proceedings of the 8th Intemationht Symposium on Combustion (Wiim & Wiiins. Baltimore, 1962). [S] T. Berces and A.F. Trotman-Dickenson, J. Chem. Sot. (1961) 384; R. Shaw and A.F. Trotman-Dickenson, J. Chem. Sot. (1960) 3210. [9] Ed. Lissi. C. Massiff and A.E. Viiia, J. Chem. Sot. Faraday Trans. I 69 (1973) 346. [IO] E.A. Lissi, G. Massiff and A.E. Vi. Intern. J. Chem. Kinet. 7 (1975) 625. [ 1 l] J.C.J. Thynne and P. Gray, Trans. Faraday Sot. 59 (1963) 1149. [12] N. Kelly and J. Heicklen, J. Photochem. 8 (1978) 83; B.A. DeCraff and J.G. Calvert, J. Am. Chem. Sot. 89 (1967) 2247. [13] G. Inoue. H. Akimoto and M. Okuda, Chem. Phys. Letters 63 (1979) 213; Abstracts of the ‘4th International Symposium on Free Radicals, Sanda. Hyogo-ken. Japan, 3-7 September i979, pp. 103-105. [14] A.P. Baronavski and J.R. hIcDonald, Chem. Phys. Letters 45 (1977) 172; V.M. Donnelly and L. Pastemack;Chem. Phys. 39 (1979) 427: A.P. Baronavski and J.R W.M. Pitt% V.M. DoMeUY, McDonald, to be published; JE. Butler, L.P. Goss, hi.C. Lin and J.W. Hudgens, Chem. Phys. Letters 63 (1979) 104; G.K. Smith, JB. Butler and MC. Lin, Chem. Phys. Letters 6.5 (1979) 115. [15] K. Ohbayashb H. Akimoto and I. Tanaka, J. Phys. Chem. 81 (1977) 798. [16] H.E. Hunziker. Abstracts of 14th International Symposium on Free Radicals, Sanda, Hyogo-ken, Japan, 3-7 September 1979, pp_ 84-87. [17] J-G. Calvert and J.N. Pitts, Photochemistry (Wiley, New York, 1966). [ 181 F. Yamada. T. Ishiwata, M. Kawasaki, K. Obi and I. Tanaka, J. Photochem. 9 (1978) 309. [19] F.W. Daiby, Can. J. Phys. 36 (1958) 1336. [20] C.H. Purkis and H.W. Thompson, Trans. Faraday Sot. 32 (1936) 1466. [21] H.W. Brown and C.C. Pimentel. J. Chem. Phys. 29 (1958) 883. [22] EA. Arden, L. Philhps and R. Shaw, J. Chem. Sot (1964) 5126. [231 H.S. Johnston, JN. Pitts. S. Lewis, L Zafonte and T. Mottersnead, Project Clean Air Task Force No. 7, University of California, 1970, pp. 3-6.